1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly to a color cathode ray tube having an electron gun which is capable of obtaining a favorable focusing in a wide phosphor screen without increasing a focus voltage which controls the correction of astigmatism associated with the deflection of electron beams and the correction of image curvature.
2. Related Art
In a cathode ray tube such as a television picture tube, a monitor tube of an information terminal equipment, other display tube or the like, electron beams emitted from an electron gun scan a phosphor screen on which a phosphor is formed (hereinafter, sometimes simply called xe2x80x9cscreenxe2x80x9d) in two directions consisting of a horizontal direction and a vertical direction to form given images.
With respect to an electron gun used in this type of color cathode ray tube, to obtain the favorable focus characteristics on the entire region of the phosphor screen, it is necessary to perform the control of shape of beam spots landed on the phosphor screen corresponding to the deflection angle of emitted electron beams.
Recently, a monitor or a television picture tube which mounts a flat tube having an outer surface of a panel thereof flattened (flat-panel type color cathode ray tube) has been commercialized. Particularly, with respect to a flat tube having a large screen which has an effective diameter of 51 cm or the like in the diagonal direction, the focusing difference between the central portion and the peripheral portion of the screen becomes large.
As a countermeasure to decrease this focusing difference, there has been known a method in which a focus electrode which constitutes an electron gun is divided into a plurality of electrode members and a focus voltage of a fixed voltage and other focus voltage which is produced by superposing a dynamic voltage which is changed in synchronism with a deflection quantity to the fixed voltage are applied to the focus electrode to form an electrostatic quadrupole lens and a curvature-of-image-field correction lens whereby the deterioration of focusing in the periphery of the screen derived from the increase of the deflection angle can be reduced.
FIG. 19 is a schematic view for explaining a general lens constitution of an electron gun which is applied to a color cathode ray tube. In the drawing, BS indicates a beam generating part, PFL indicates a prefocus lens, FL indicates a front-stage main focus lens, IL indicates a curvature-of-image-field correction lens, ML indicates a rear-stage main focus lens (also called xe2x80x9cfinal-stage main focus lens), and SC indicates a phosphor screen.
Respective lenses described above are arranged in the direction of the phosphor screen SC from the beam generating part BS side along a tube axis Zxe2x80x94Z These lenses focus electron beams B generated by the beam generating part BS, then accelerate the electron beams B and finally make the electron beams B impinge on the phosphor screen SC so as to form electron beam spots (simply called xe2x80x9cbeam spotsxe2x80x9d hereinafter).
To be more specific, the above-mentioned electron gun is constituted by the beam generating part (triode part) which is constituted by a cathode (usually called xe2x80x9cKxe2x80x9d), a control electrode (usually called xe2x80x9cG1xe2x80x9d) and an accelerating electrode (usually called xe2x80x9cG2xe2x80x9d) and generates a plurality of electron beams, and a main lens part which is made of focus electrodes (usually called xe2x80x9cG3xe2x80x9d, xe2x80x9cG4xe2x80x9d xe2x80x9cG5xe2x80x9d) and an anode (usually called xe2x80x9cG6xe2x80x9d) and focus the electron beams generated by the beam generating part toward the phosphor screen.
Here, the electron gun adopts a multi-stage dynamic focusing (MDF) system where the focus electrode (G5) is divided into a plurality of electrode members. By applying a fixed focus voltage and a dynamic correction voltage which is produced by superposing a dynamic voltage which is changed in synchronism with a deflection quantity to the divided electrode members, an electrostatic quadrupole lens and a curvature-of-image-field correction lens which are provided for ensuring desired focusing characteristics in a wide range of the phosphor screen are formed. Most of the conventional electron guns adopt the non-multi-stage dynamic focusing.
FIG. 20 is an explanatory view of the focus voltage applied to the focus electrode divided into a plurality of electrode members. Further, FIG. 21 is an explanatory view of an output voltage of a flyback transformer which generates two focus voltages.
As shown in FIG. 20, the focus electrode G5 of the electron gun is divided in multi-stages (here, three stages consisting of electrode members A, B and C) so as to constitute an electron gun of a composite lens type and the electrostatic quadrupole lens and the curvature-of-image-field correction lens are formed among the electrode members A, B and C. The curvature-of-image-field correction lens is provided for correcting the difference of distance from the center of deflection to the phosphor screen and is usually arranged at a position closer to the phosphor screen than the electrostatic quadrupole lens.
The electrostatic quadrupole lens controls the cross section of the beam spots which pass through the electrostatic quadrupole lens so as to reduce the shape of the beam spot on a phosphor screen into a shape similar to a circle.
The first fixed voltage Vf1 is applied to the electrode member B and other focus voltage (Vf2+dVf) which is produced by superposing a dynamic voltage dVf which is changed in synchronism with a deflection quantity to the second fixed voltage Vf2 is applied to the electrode members A and C.
The above-mentioned focus voltages Vf1, Vf2+dVf are generated by the flyback transformer FBT shown in FIG. 21. Here, Eb indicates an anode voltage (maximum voltage) which is applied to the anode G6, Ec2 indicates a prefocus voltage of approximately 600V applied to other electrodes (G2, G4) of the electron gun.
FIG. 22 is an explanatory view of the focus voltage applied to the electrode members of the divided focus electrode, wherein 1V indicates 1 vertical deflection cycle (1 frame cycle or 1 field cycle) and 1H indicates 1 horizontal deflection cycle.
When the dynamic voltage dVf is increased, that is, when the deflection quantity of the electron beams is large (at the time of deflecting the electron beams toward the peripheral portion of the screen), the potential difference at the curvature-of-image-field correction lens becomes small so that the intensity of the lens is decreased. Accordingly, the force to focus the electron beams becomes weak at the time of deflecting the electron beams so that the image curvature is corrected.
This type of conventional technique is, for example, disclosed in Japanese Laid-open Patent Publication 43532/1992 and Japanese Laid-open Patent Publication 161309/1995.
With respect to the conventional technique, particularly Japanese Laid-open Patent Publication 43532/1992, a focus electrode disposed close to an anode is divided into a plurality of first electrode members and a plurality of second electrode members, wherein the first electrode member and the second electrode member are alternately arranged in the electron beam advancing direction. Then, the first electrode member and the second electrode member form a curvature-of-image-field correction lens in the state that the first electrode member and the second electrode member are made electrically independent from each other to form an electron lens which changes the intensity thereof in synchronism with the deflection of the beams between the first electrode member and the second electrode member.
Further, a non-axially-symmetric electron lens for correcting astigmatism which deforms the cross-sectional shape of the electron beams due to the above-mentioned fluctuating dynamic voltage is formed adjacent to a main lens so that even when the fluctuation of the focus voltages is suppressed at a low level, a favorable image can be obtained on the whole screen.
However, the electron gun which uses the multi-stage focus electrode has the total length thereof elongated so that although the diameter of the beam spots on the screen becomes small, it is necessary to increase the focus voltage. For example, with respect to a flat type color cathode ray tube having a screen diagonal dimension of 51 cm and a deflection angle of 90 degrees, when the length of the focus electrode is increased by 1 mm, the focus voltage is elevated by approximately 0.36%.
Although the focus voltage is generated by the flyback transformer, usually the rated output voltage range of the flyback transformer which is used as a power supply of the cathode ray tube of this type is approximately 28%xc2x12% of an anode voltage. Accordingly, when the focus voltage is increased by elongating the focus electrode, the flyback transformer of a general use can not cope with the increased focus voltage. Therefore, the lowering of the focus voltage has been one of the tasks to be solved by the present invention.
It is a typical object of the present invention to provide a color cathode ray tube having an electron gun which improves the focusing characteristics in a wide region of a phosphor screen by setting the total length of a focus electrode divided in multi-stages within a given value and by properly selecting the mounting position and the sensitivity of an electrostatic quadrupole lens.
To achieve the above-mentioned object, according to a first aspect of the present invention, in a typical constitution of the present invention, a focus electrode includes a plurality of electrode members which constitute an electrostatic quadrupole lens which changes the cross-sectional shape of electron beams in synchronism with the deflection of the electron beams and an electron lens whose focusing force is fluctuated in synchronism with the deflection of the electron beams, and assuming the distance from an anode-side end portion of the focus electrode to an anode-side end portion of the electrostatic quadrupole lens as L2 (mm), the relationship of 7.55xe2x89xa6L2xe2x89xa611.5 is set.
According to a second aspect of the present invention, with respect to the above-mentioned focus electrode, in a surface of one electrode member which constitutes the electrostatic quadrupole lens and faces the other electrode member in an opposed manner, longitudinally elongated electron beam passing apertures which have a long axis in the vertical direction are formed,
on a surface of the other electrode member which constitutes the electrostatic quadrupole lens and faces one electrode member in an opposed manner, a plural pairs of horizontal correction electrode plates are formed such that the electrode plates sandwich a plurality of respective electron beams from the vertical direction, the electrode plates are protruded in the tube axis direction toward one electrode member, and the electrode plates make protruding ends thereof inserted into electron beam passing apertures of one electrode member in the vicinity of both ends of the apertures in the long axis direction, and
assuming the electrode length in the tube axis direction of the horizontal correction electrode plates as L5 and the distance in the vertical direction of a pair of horizontal correction electrode plates as L6, the relationship of 0.0206xe2x89xa6L5/(L62.7)xe2x89xa60.0306 is set.
According to a third aspect of the present invention, with respect to the above-mentioned focus electrode, in a surface of one electrode member which constitutes the electrostatic quadrupole lens and faces the other electrode member in an opposed manner, longitudinally elongated electron beam passing apertures which have a long axis in the vertical direction are formed,
in a surface of the other electrode member which forms the electrostatic quadrupole lens and faces one electrode member in an opposed manner, a laterally elongated electron beam passing aperture which has a horizontal long axis is formed, and
assuming the distance from the surface of the focus electrode which faces the anode in an opposed manner to the anode-side position of the electrostatic quadrupole lens as L2 (mm), the relationship of 7.55xe2x89xa6L2xe2x89xa611.5 is set.
According to a fourth aspect of the present invention, with respect to the above-mentioned focus electrode, on a surface of one electrode member which constitutes the electrostatic quadrupole lens and faces the other electrode member in an opposed manner, vertical correction electrode plates which sandwich a plurality of respective electron beams from the horizontal direction and are protruded along the tube axis toward the opposing other electrode member are formed, and
on a surface of the other electrode member which constitutes the electrostatic quadrupole lens and faces one electrode member in an opposed manner, horizontal correction electrode plates which sandwich a plurality of respective electron beams from the vertical direction, are protruded along the tube axis toward one electrode member and are superposed with the vertical correction electrode plates are formed, and
assuming the electrode length in the tube axis direction of the vertical correction electrode plates as L3 and the electrode length in the tube axis direction of the horizontal correction electrode plates as L4, the relationship of 2.18xe2x89xa6(L3+L4)/2xe2x89xa62.78 is set.
According to a fifth aspect of the present invention, when the distance between a surface of one electrode member which forms the electrostatic quadrupole lens and faces the other electrode member in an opposed manner and a surface of the other electrode member which forms the electrostatic quadrupole lens and faces one electrode member in an opposed manner is set to not more than 1 mm, or when the width of end portions in the long axis direction (longitudinally up-and-down direction) of the longitudinally elongated electron beam passing apertures formed in the surface of one electrode member which faces the other electrode member in an opposed manner is set to W1 and the width of end portions in the long axis direction (laterally left-and-right direction) of the laterally elongated electron beam passing apertures formed in the surface of the other electrode member which faces one electrode member in an opposed manner is set to W2, the relationship of 2.00xe2x89xa6(W1+W2)/2xe2x89xa63.60 is set.
Due to the above-mentioned constitution, it becomes possible to obtain a favorable focusing in a wide range of current area and in a wide range of screen area. Further, in the limited total length of the focus electrode, the mounting position and the sensitivity of the electrostatic quadrupole lens can be properly set and hence, the focusing characteristics of the electron gun can be improved in a wide area of the phosphor screen.
The present invention is not limited to the above-mentioned constitution and the constitutions of embodiments which are explained hereinafter and various modification are conceivable without departing from the technical concept of the present invention.